With increasing development of three-dimensional (3D) imagery technology, various stereoscopic image products are gradually introduced into the market. In the conventional polarization conversion system, different images are received by the left eye and the right eye, and synthesized to a stereoscopic image.
In accordance with a current stereoscopic projection approach (e.g. an active approach), the user may wear liquid crystal shutter glasses to actively receive the stereoscopic image. The glass for the left eye and the glass for the right eye are turned on and turned off at different time spots, so that the stereoscopic image light is alternately directed to the left eye and the right eye. That is, the images with time difference are respectively received by the left eye and the right eye, and then synthesized to a stereoscopic image. However, this approach has some drawbacks. For example, the liquid crystal shutter glasses are bulky and heavy. In addition, it is inconvenient to periodically replace the battery of the liquid crystal shutter glasses.
In accordance with another stereoscopic projection approach (e.g. a passive approach), the stereoscopic image is passively received. That is, the stereoscopic image light is split into a P-polarized beam and an S-polarized beam. By wearing lightweight 3D image glasses with different polarizer plates allowing the image of one polarization to pass to the left eye and the image of the orthogonal polarization to pass to the right eye, different polarized beams (P-polarized beams and S-polarized beam) on the projection screen are synthesized to a stereoscopic image.
U.S. Pat. No. 7,857,455 discloses a stereoscopic projection system by combining a P-polarized beam and an S-polarized beam. FIG. 1 schematically illustrates a conventional stereoscopic projection system by combining a P-polarized beam and an S-polarized beam. As shown in FIG. 1, the stereoscopic projection system 1 comprises a polarization beam splitter 103 for receiving image light energy from a projection lens 102 and splitting the image light energy into a primary path and a secondary path. A reflective mirror 106 is located in the secondary path. In addition, a polarization modulator 104 is positioned in the primary path to rotate P-polarized beam into S-polarized beam, and another polarization modulator 105 is positioned in the secondary path for polarization rotation in the secondary path. After the light from a light source (not shown) is modulated by an imaging surface 101 and transmitted through the projection lens 102, the stereoscopic image light split into the primary path and the secondary path. A P-polarized beam 107 along the primary path is modulated by the polarization modulator 104. An S-polarized beam 108 along the secondary path is modulated by the polarization modulator 105, and reflected by the reflective mirror 106. In such way, the modulated P-polarized beam 107 and modulated S-polarized beam 108 are turned into the same polarization and projected on the same position of a projection screen 109.
Another polarization conversion system for a stereoscopic projection system is disclosed in U.S. Pat. No. 7,905,602. FIG. 2 schematically illustrates a conventional polarization conversion system for a stereoscopic projection system. The polarization conversion system (PCS) is included in the stereoscopic projection system 2. As shown in FIG. 2, the polarization conversion system comprises a polarization beam splitter (PBS) 202, a polarization rotating element 203, a reflective element 204 and a polarization switch 205. After the stereoscopic image light from a projection lens 201 is directed to the polarization conversion system, the stereoscopic image light is split by the polarization beam splitter 202 a P-polarized beam 206 and an S-polarized beam 207. The P-polarized beam 206 is directly transmitted through the polarization switch 205 along a first optical path. The S-polarized beam 207 is directed to the polarization rotating element 203 along a second optical path, and transformed into a P-polarized beam 208 by the polarization rotating element 203. The P-polarized beam 208 is reflected by the reflective element 204, and then transmitted through the polarization switch 205 along the second optical path. By the polarization switch 205, the polarized beams from the first optical path and the second optical path can be switched between the P-polarized beam and the S-polarized beam. Afterwards, the stereoscopic image light is directed toward a projection screen 209 along the first optical path and the second optical path, so that a brighter image is projected on the projection screen 209. Although the use of either the stereoscopic projection system 1 or the stereoscopic projection system 2 can achieve a brighter image output, there are still some drawbacks. For example, since the polarization conversion system is positioned downstream of the projection lens 201 and located where the image light is divergent, the polarization beam splitter 202, the polarization rotating element 203, the reflective element 204 and associated components are relatively bulky. Consequently, the overall volume of the polarization conversion system is large and the fabricating cost is increased. In addition, when the stereoscopic image light passes through the polarization beam splitter 202, the image quality is deteriorated.